cmd/go/internal/modfetch: run git log with "-c log.showsignature=false"

The old version git not support "--no-show-signature", git add this from
v2.10.0.

Fixes golang/go#26501.

Change-Id: Ia6b54488651e8687b08a4d40e092822bf960c4fe
Reviewed-on: https://go-review.googlesource.com/125315
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modfetch/codehost/git.go

@@ -378,7 +378,7 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 // statLocal returns a RevInfo describing rev in the local git repository.
 // It uses version as info.Version.
 func (r *gitRepo) statLocal(version, rev string) (*RevInfo, error) {
-	out, err := Run(r.dir, "git", "log", "-n1", "--format=format:%H %ct %D", "--no-show-signature", rev)
+	out, err := Run(r.dir, "git", "-c", "log.showsignature=false", "log", "-n1", "--format=format:%H %ct %D", rev)
 	if err != nil {
 		return nil, fmt.Errorf("unknown revision %s", rev)
 	}